Three Plan­ets Bask in the Hab­it­able Zone

Trappist-1 is only 0.05% as bright as the Sun, but as the so­lar sys­tem is very small, three of the plan­ets still or­bit in the hab­it­able zone.

Science Illustrated - - SPACE -

"For decades now, we have been liv­ing on the planet of Nuwa in a small so­lar sys­tem with seven rocky plan­ets. On rare oc­ca­sions, vi­o­lent storms blow holes in the thick cloud cover, which usu­ally shrouds the world of Shen­nong. When I was 20 years old, I came close to the planet, trav­el­ling in one of our space­craft, and I saw some­thing in­cred­i­ble through a hole in the clouds. A tall struc­ture mea­sur­ing sev­eral km, per­haps a space el­e­va­tor or an at­mo­spheric power sta­tion. Ever since, I have dreamt of go­ing to that veiled planet one day. We do not dare to yet, as we can­not see the sur­face. But they are there. Their civil­i­sa­tion is an­cient and ex­tremely so­phis­ti­cated. They must have ob­served us many years ago, as we ap­proached the plan­e­tary sys­tem aboard our space­craft.”

The ac­count is pure sci­ence fic­tion, writ­ten by Swiss au­thor Lau­rence Suh­ner, but the text is sen­sa­tional, as it was pub­lished in one of the world’s lead­ing sci­en­tific jour­nals, Na­ture, which usu­ally keeps straight to the facts. But now, imag­i­na­tion is al­lowed to rule be­cause of ex­cite­ment con­cern­ing the dis­cov­ery of seven Earth-like rocky plan­ets, which are or­bit­ing a small red dwarf star only 39 light years from Earth. And three of the plan­ets are even lo­cated in the hab­it­able zone, where wa­ter could flow on their sur­faces, so they might in­clude life. Hence, when the dis­cov­ery was pub­lished, Na­ture asked the writer to leap 400 years for­ward in time, imag­in­ing what life would be like in a colony near one of the plan­ets.


The dwarf star has been known since 1999, but did not be­come re­ally in­ter­est­ing un­til last year, when Michaël Gil­lon and his col­leagues from the Univer­sity of Liège, Bel­gium, aimed the 60 cm Trappist telescope at the red dwarf. The telescope is de­signed to look for ex­o­plan­ets, which re­veal their ex­is­tence, when they pass in front of the star as ob­served from Earth. As it passes by the star, the planet re­duces the light of the star, and the telescope

reg­is­ters the light re­duc­tion. The first ob­ser­va­tions showed that two Earth-like plan­ets were or­bit­ing close to the star, whereas yet another planet was or­bit­ing fur­ther away. The star was named Trappist-1, and the Bel­gian astronomers were al­lowed to use the Spitzer space telescope for 500 hours of fur­ther stud­ies.

The ob­ser­va­tions paid off. The telescope re­vealed that a to­tal of seven rocky plan­ets or­bit the dwarf star.

Five of the plan­ets are the size of Earth, whereas the two last ones are smaller, sized some­where in be­tween Earth and Mars.

The seven are or­bit­ing the star so close to­gether that the dis­tance be­tween the two in­ner­most plan­ets is only 1.6 times the dis­tance be­tween Earth and the Moon. The in­ner­most planet only takes 1.5 days to com­plete an or­bit, whereas the out­er­most planet or­bits the star in a mat­ter of 19 days.

In com­par­i­son, the in­ner­most planet of the So­lar Sys­tem, Mer­cury, takes 88 days to or­bit the Sun.


The tiny sys­tem is only slightly big­ger than Jupiter and its four largest moons – Io, Europa, Ganymede, and Cal­listo. The prox­im­ity of the dwarf star means that three of the plan­ets might in­clude life. The red dwarf only shines 0.05 % as brightly as the Sun, but that is more than enough to make the three in­ner­most plan­ets too hot. The three next ones are lo­cated in the hab­it­able zone, where liq­uid wa­ter could ex­ist. The out­er­most planet is prob­a­bly frozen.

The great sim­i­lar­ity to Jupiter’s moons has con­vinced the Bel­gian astronomers that the plan­e­tary sys­tem was formed in the same way as they were. Con­se­quently, the new­born dwarf star was sur­rounded by a disc of dust and gas, which was so dense that it gave birth to sev­eral gen­er­a­tions of plan­ets. To­gether with lots of dust, they spi­ralled into the star and were swal­lowed one by one. Not un­til the disc had be­come so "thin" that the grav­ity of the dust no longer con­trib­uted to at­tract­ing the plan­ets to the star, the ex­ist­ing plan­ets were formed. At the same time, the plan­ets’ mu­tual at­trac­tion

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